I can't point to any scientific studies, but I find it a lot easier to read old Lisp code that I've written than old Java code that I've written. Of course, there's not all that much old Java code that I've written, so it may just be a matter of familiarity, which may also explain why you believe Lisp to be hard to read. I suspect part of it, though, is that Lisp lets me more directly translate how I think about a problem into its solution as a program, which means that the program, viewed at a later time, will more naturally correspond with what is in my mind, even at that later time.